Vlasov – Maxwell, Self-consistent Electromagnetic Wave Emission Simulations in the Solar Corona

Abstract

1.5D Vlasov-Maxwell simulations are employed to model electromagnetic emission generation in a fully self-consistent plasma kinetic model for the first time in the solar physics context. The simulations mimic the plasma emission mechanism and Larmor drift instability in a plasma thread that connects the Sun to Earth with the spatial scales compressed appropriately. The effects of spatial density gradients on the generation of electromagnetic radiation are investigated. It is shown that 1.5D inhomogeneous plasma with a uniform background magnetic field directed transverse to the density gradient is aperiodically unstable to Larmor-drift instability. The latter results in a novel effect of generation of electromagnetic emission at plasma frequency. The generated perturbations consist of two parts: (i) non-escaping (trapped) Langmuir type oscillations which are localised in the regions of density inhomogeneity, and are highly filamentary, with the period of appearance of the filaments close to electron plasma frequency in the dense regions; and (ii) escaping electromagnetic radiation with phase speeds close to the speed of light. When density gradient is removed (i.e. when plasma becomes stable to Larmor-drift instability) and a low density super-thermal, hot beam is injected along the domain, in the direction perpendicular to the magnetic field, plasma emission mechanism generates non-escaping Langmuir type oscillations which in turn generate escaping electromagnetic radiation. It is found that in the spatial location where the beam is injected, the standing waves, oscillating at the plasma frequency, are excited. These can be used to interpret the horizontal strips (the narrowband line emission) observed in some dynamical spectra. Quasilinear theory predictions: (i) the electron free streaming and (ii) the beam long relaxation time, in accord with the analytic expressions, are corroborated via direct, fully-kinetic simulation. Finally, the interplay of Larmordrift instability and plasma emission mechanism is studied by considering dense electron beam in the Larmor-drift unstable (inhomogeneous) plasma. The latter case enables one to study the deviations from the quasilinear theory.

An alternative to the plasma emission model: Particle-in-cell, self-consistent electromagnetic wave emission simulations of solar type III radio bursts

“…In Ref. [8] it was shown that when an electron beam, with the properties similar to considered here, is injected perpendicular to the background magnetic field, the beam excites electrostatic, standing waves, oscillating at local plasma frequency, in the beam injection spatial location. Here physical situation is different in that now the beam is injected along the magnetic field which is more plausible for the type III radio bursts.…”

“…Thus, we conclude that the equilibrium without the electron beam is fairly stable (apart from the low level EM drift wave noise). In this section we present the results when we inject the electron beam along the background magnetic field [8], when the beam is relatively dense (n b /n e ≈ few 10 −2 ), the electron number density perturbation is dominated by a wake created by the beam. In Ref.…”

“…We refer the interested reader to appropriate reviews [4][5][6][7] and to references in Ref. [8]. Also Introduction section of Ref.…”

“…The main results of Ref. [8] can be summarised as following: In the case without an electron beam, the induced perturbations consist of two parts: (i) non-escaping Langmuir-type oscillations which are localised in the regions of density inhomogeneity, and are highly filamentary, with the period of appearance of the filaments close to electron plasma frequency in the dense regions; and (ii) escaping EM radiation with phase speeds close to the speed of light. When we removed the density gradient (i.e.…”

“…Another way to look at the case considered in Ref. [8] is to realise that v b × B z0 = E (because the resistivity is zero and plasma beta is small, electrons tend to be magnetised, 'frozen-into' plasma). In this case the latter vector product gives v x B z0 = E ⊥ = E y .…”